ABSTRACT: Percentage compositions by weight and by number are the frequently used measures to evaluate the relative importance of different prey types in fish diet studies. The results obtained using these measures are only point estimates, and are often
reported without any indication of their precision. Here an attempt is made to set confidence limits to these estimates via normal approximation and bootstrapping. Applications of these approaches are demonstrated with the stomach contents data of
mackerel from the North Sea. The precision estimates of the average weight percentages were generally very poor, even when the prey items were grouped into major categories and sample sizes were comparatively large. Intra-haul correlation was found to be
an important source of variation in the stomach content composition. A bootstrap method, which incorporated inter- and intra-haul variation, provided more realistic confidence intervals for weight percentages than the normal approximation. Implications of
the uncertainty associated with these measures for dietary studies have been discussed. Routine estimation of the precision of dietary measures is recommended.